High intensity arc discharge lamps such as metal halide lamps and mercury lamps typically include a quartz arc tube mounted within a glass outer envelope. In some cases, the region between the arc tube and the outer envelope is filled with an inert gas such as nitrogen, while in other cases this region is evacuated. The radiation generated by arc discharge lamps contains potentially harmful ultraviolet radiation which is blocked by the glass outer envelope.
In one failure mode, the arc tube bursts, thereby terminating emission of radiation. Various techniques have been disclosed in the prior art for containing the fragments of an arc tube which bursts, and for insuring that the outer envelope remains intact. Such techniques include the use of a thick walled outer envelope and the use of a light-transmissive shroud between the arc tube and the outer envelope.
In another failure mode, the outer envelope of the arc discharge lamp is broken by an external impact. In this case, the arc tube may continue operating and emitting potentially harmful ultraviolet radiation which is no longer blocked by the outer envelope. Frequently, arc discharge lamps are operated in enclosed fixtures which contain fragments of a shattered outer envelope and absorb ultraviolet radiation. However, in other applications, it is desirable to operate arc discharge lamps in open fixtures which are generally less expensive than enclosed fixtures and may be preferable for technical and/or aesthetic reasons.
To prevent operation of the arc tube in the event of an outer envelope failure, a low wattage, easily-oxidized safety filament is sometimes included in the lamp. The safety filament is located within the outer envelope and is electrically connected in series with the arc tube. If the outer envelope is broken, the safety filament is rapidly oxidized when it comes in contact with the oxygen in the air, thereby interrupting the electrical circuit of the arc tube and extinguishing the lamp. This technique is disclosed in European Patent Application No. 0,326,079. Techniques for extinguishing arc discharge lamps when the outer envelope is broken are also disclosed in U.S. Pat. No. 4,013,919 issued Mar. 22, 1977 to Corbley, U.S. Pat. No. 4,013,920 issued Mar. 22, 1977 to Petro, U.S. Pat. No. 4,208,614 issued June 17, 1980 to Strauss et al and U.S. Pat. No. 4,629,939 issued Dec. 16, 1986 to Jaworowicz et al.
It has been observed that the inclusion of a safety filament in a metal halide arc discharge lamp of the type disclosed in U.S. Pat. No. 4,888,517 issued Dec. 19, 1989 to Keeffe et al and containing sodium iodide and scandium iodide, considerably reduces the operating life of the lamp. The lamp disclosed in the Keeffe et al patent includes a shroud surrounding the arc tube and a frame for mechanically supporting the shroud and the arc tube. When a safety filament is included in such a lamp, excessive arc tube voltage rise and changes in the color temperature of the lamp are observed in a relatively short time. It is desirable to overcome such problems and to provide an arc discharge lamp having a safety filament for protection in the event of outer envelope breakage, and having a long operating life.
It is a general object of the present invention to provide improved arc discharge lamps.
It is another object of the present invention to provide double enveloped arc discharge lamps having a safety filament and having a long operating life.
It is a further object of the present invention to provide double-enveloped arc discharge lamps having a safety filament provided with an insulating sleeve for suppressing emission of electrons.
It is still another object of the present invention to provide arc discharge lamps wherein migration of sodium and other alkali metal ions from the arc tube is suppressed, thereby extending the operating life of the lamp.
It is a further object of the present invention to provide arc discharge lamps that are safe in the event of outer envelope breakage.
It is another object of the present invention to provide double enveloped arc discharge lamps that are extinguished within a prescribed time after the outer envelope is broken.